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In other sections of the site's RAID coverage, I have hinted at some of the
considerations that go into selecting drives for a RAID array. Some of these are fairly
obvious, but others are not. Here is a fairly comprehensive list of issues to keep in mind
when considering drives for a RAID array:

Interface: You must of course get drives that correspond to the
interface your controller uses (really, this is a "group decision" of course.)

Capacity: Get the largest drives
that you can afford and that will physically fit your system or enclosure.

Physical Size: Don't get half-height
drives unless you have a case or enclosure that can handle them.

Performance: Get the fastest drives you can afford. Hard disk
performance is a huge subject unto itself, which I
won't try to summarize in a paragraph here, other than to say that if performance is
important to you, you must educate yourself about performance issues and choose drives
that meet your needs. The only synopsis I'll provide is to say that if your main concern
is random access performance, look for a drive model with low overall access time; if transfer rate is of utmost
importance, instead look for drives with the highest sustained transfer rates.

Source: If you are buying a "pre-made" array, get it from a
high-quality manufacturer or system integrator. If you are setting up the array yourself, always
buy drives from a reputable source, preferably an authorized dealer. This will ensure that
you get a proper warranty, that you have support if you need it, and that you don't end up
buying second-rate merchandise.

Uniformity: To keep performance balanced and to maximize storage
efficiency, drives should all be the same size and model. However, also keep in
mind the point below...

Diversification: This is a very important consideration when selecting
drives for a RAID array. Drive failures are sometimes caused by random flaws, making an
individual unit fail while others made at the same time are just fine. But sometimes,
drive failures are related to occasional manufacturing difficulties; this can cause a
specific batch of drives to have a much higher rate of failure than normal for that drive
family. If you set up six drives in a RAID 5 array and they are all from a faulty lot, you
run the very real risk of having two simultaneous failures and losing the entire
array. For this reason, you should get drives that are all the same make, model and size,
but you should if at all possible get drives from different manufacturer lots. A good
vendor should be able to help you do this.